The present invention relates to a method of preparing .beta.-alkoxybutyric acid alkyl esters by the reaction of allyl halides with carbon monoxide, alkali alcoholates and alcohols in the presence of catalysts.
It is known to prepare vinyl acetic acid methyl esters plus small amounts of crotonic acid methyl esters from allyl halides with carbon monoxide and methanol in the presence of catalytic amounts of nickel carbonyl and thiourea, at atmospheric pressure, at a temperature between 15 and 35.degree.C, and at a pH between 5.5 and 9 (German "Offenlegungsschrift" No. 1,936,725).
It is furthermore known that palladium chloride catalyzes the carbonylation of allyl chloride. In the presence of an inert solvent such as benzene, vinyl acetyl chloride forms; in an alcohol as solvent, the corresponding vinyl acetic acid esters form. Crotonic acid chloride or crotonic acid esters occur as by-products due to isomerization of the double bond (J. Tsuji, J. Kiji, S. Imanura and M. Morikawa, J. Amer. Chem. Soc. 86, 4350 (1964) and D. Medema, R. Van Helden and C. F. Kohll, Inorg. Chem. Acta, 3, 255 (1969)).
Disadvantages of these processes lie in the necessity of using carbon monoxide in the 100 kp/cm.sup.2 pressure range, and in the unfavorable weight ratio of the allyl halide to palladium chloride, long reaction times or low yields, and reactions performed in a plurality of stages. .pi.-allyl metal complex compounds are formed as intermediates in the carbonylation of allyl halides, namely bis-.pi.-allyl nickel bromide and .pi.-allyl palladium bromide, which by reaction with CO and methanol produce vinyl acetic acid or crotonic acid methyl ester, respectively (Z. Naturforschung 17b (1962) 484 and 850), or, in the latter case, by reaction with CO and ethanol, have produced vinyl acetic acid ethyl ester (Tetrahedron Letters 26 (1963) 1811).
Furthermore, .beta.-alkoxybutyric acid alkyl esters can by prepared from crotonic acid esters and alcohol under the catalytic influence of alkali alcoholates (Ullmann (1960), Vol. 5, p. 617).